A cruise-control system compares the present speed with a desired speed. If the present speed is lower than the desired speed, the car speed is increased. If the present speed is higher than the desired speed, the vehicle is decelerated. In recent years, automobiles have been required to have numerous functions or have high efficiency. Thus, automobiles must cope with such complex situations.
As an example, an electronic fuel-injection system is used. The electronic fuel-injection system employs a microcomputer or other electronic control unit to control the amount of fuel supplied to the engine by adjusting the opening of the throttle valve, the flow of air, or the engine speed.
One method of controlling fuel supply by an electronic fuel-injection system consists in cutting off the supply of fuel from the throttle valve when the engine speed is in excess of a certain valve and when the valve is at idle throttle. This is effective in improving the braking force of the engine and enhancing the fuel consumption ratio. Unfortunately, if this method is combined with a cruise-control system, the following problems take place.
A cruise-control system closes the throttle to prevent the vehicle speed from increasing. When the vehicle coasts down a steep hill, the throttle may be fully closed. At this time, it is necessary to cut off the supply of fuel and so the electronic fuel-injection system performs this job. If so, the engine brakes the vehicle, so that the vehicle speed drops rapidly. Then, the cruise-control system opens the throttle to suppress the reduction in the vehicle speed. This, in turn, permits the electronic fuel-injection system to supply fuel. This resupply of fuel increases the vehicle speed again. In this way, the aforementioned steps are repeated. As a result, the vehicle speed surges. This makes the passengers very unpleasant.
In an attempt to solve the foregoing problems, Japanese Patent Laid-Open No. 11444/1988 discloses a new cruise-control system. Once the operating conditions require a fuel reduction while the vehicle speed is maintained by this cruise-control system, the fuel reduction is continued as long as the difference between the intended vehicle speed and the present speed is less than a predetermined level even if the aforementioned operating conditions no longer hold.
Japanese Patent Laid-Open No. 135334/1985 discloses another cruise-control system. When a desired speed is maintained by this system, if the operating conditions permit a fuel reduction, then the supply of fuel is decreased. Subsequently, the loop gain for the cruise control is set to a small value for a given time. This prolongs the period during which the fuel cutoff is not permitted by the operating conditions. That is, the supply of fuel is cut off at longer intervals of time. Hence, the passengers will feel less unpleasant.
Japanese Patent Laid-Open No. 196828/1986 discloses a further cruise-control system. This system stops the supply of fuel if the throttle is fully closed and if the present vehicle speed is larger than the desired speed by a predetermined value, to prevent the vehicle speed becoming much larger than the desired speed when the vehicle goes down a steep hill.
In any of the aforementioned known cruise-control systems, the response characteristics are lowered substantially in order that the operating conditions allow a fuel reduction at longer intervals of time. Although the surge occurring when the vehicle goes down a hill is reduced, it is not prevented. Therefore, the passengers still feel uncomfortable. Specifically, depending on the set reference value from which the speed difference is calculated, on the loop gain, or on the steepness of the hill, fuel reduction is caused by the operating conditions at the same intervals of time, thus producing uncomfortable surge. Conversely, when fuel cut is made unnecessary, it still continues.